mmc: sirf: fix sdhci_pltfm_init sequence

Patch "mmc: sdhci: Add size for caller in init+register" changed the
interface for sdhci_pltfm_init, while patch "mmc: sdhci-sirf: add mmc
host sdhci-pltfm based driver for SiRF SoCs" added a new driver
with the old interface.

This changes the sirf driver to use the new interface, avoiding
one warning, and simplifying the init sequence. Since we're here
already, this also adds an error path for failed clk_prepare_enable.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Christian Daudt <csd@broadcom.com>
Signed-off-by: Chris Ball <cjb@laptop.org>